Its 400-acre estate on Osea Island in the Blackwater Estuary, Essex, is owned by pop group Sugababes' producer Nigel Frieda.

It opened in 2004 as the world's first and only island dedicated to treating mental health problems as well as drug and alcohol addiction.

The prosecution — by the Care Quality Commission health watchdog, which regulates NHS trusts and private clinics — centres on the treatment of patients with psychiatric problems.

The watchdog first inspected the retreat after allegations it was breaching the Care Standards Act by offering unregistered services.

The commission then requested that no new psychiatric patients be admitted but was allegedly ignored.

It ordered the retreat to remove clients or stop providing treatments it was not permitted to offer — but is said to have found after several unannounced assessments that it was still in breach of guidelines.

The island is only accessible by road across a causeway for four hours a day at low tide. Emergency medical assistance can be provided by helicopter — a transport mode favoured by many of its clients but understood to be a matter of concern for the commission.

The Causeway Retreat's chief executive Brendan Quinn is to appear at Harlow magistrates' court tomorrow when the case gets under way.